Example of environmental final exam

One of the following sentences is correct about lime/soda-ash softening process i Magnesium precipitates as MgCO3 ii After treatment the water will have a low pH value iii Soda ash will

College of Engineering

Civil and Environmental Engineering Department

CIVL375: Water and Wastewater Technology

Example Final Exam

1 (9 pts) Select the most appropriate answer

Part A through D are related to the table below which shows C•t (mg l-1 min) values for 1.0-Log (90%) inactivation of a certain microorganism

0.5 °C 5 °C 10 °C 15 °C 20 °C

Performed chloramine 1300 740 620 500 370

A The strongest disinfectant among the four listed in the above table is

(i) Free chlorine (ii) Performed chloramines

(iii) chloride dioxide (iv) Ozone

B The value of C•t (mgl-1 min) for free chlorine for 99% inactivation at 20 °C is about

C The value of C•t (mgl-1 min) for free chlorine for 90% inactivation at 25 °C is about

D A water at 20 °C is to be disinfected using ozone with 90% inactivation For a contact time of

10 min, the applied ozone dose (mg/l) should be

E One of the disadvantages of using free chlorine for disinfection is

(i) it forms THMs (ii) it forms chlorate

(iii) it is cheap (iv) it leaves residual

F One of the following sentences is correct about lime/soda-ash softening process

(i) Magnesium precipitates as MgCO3

(ii) After treatment the water will have a low pH value

(iii) Soda ash will be needed when there is not enough alkalinity in the water

J Reject brine from a distillation unit has 3% dissolved solids Thus, the concentration of

dissolved solids in the brine in mg/l is

K Applied chlorine dose versus chlorine residual of drinking water is shown below The applied chlorine dose (mg/l) needed to have a free available chlorine of 0.4 (mg/l) is

L From the previous figure, the chlorine demand at an applied chlorine dose of 1.6 mg/l is about (i) 1.6 mg/l (ii) 0.6 mg/l (iii) 1.0 mg/l (iv) 2.2 mg/l

2 (4 pts) What is the minimum diameter of a cylindrical CSTR needed to remove 95% of

a contaminant entering the reactor at a flow rate of 20 m3/hr? The reaction rate constant is 9.0/day and the reactor depth should not exceed 4.0 m.

3 (4 pts) A reverse osmosis unit (schematically shown below) is to desalinate 100,000

gal of brackish water per day The membrane mass transfer coefficient is 0.035

psi

d

ft

gal

2 , the pressure difference between the feed and the product water is 350 psi, and the osmotic pressure difference between the feed and the product water is 45 psi.

A What is the required area (ft2) of the membrane?

B What is the concentration (mg/l) of dissolved solids in the brine?

Feed:

Qf=100,000 gal/d

Cf= 10,000 mg/l

Product:

Qp=70,000 gal/d

Cp= 250 mg/l

Brine

4 (5 pts) Results of water analysis are presented in the following meq/l bar graph

Answer the following questions

a The total hardness of the water in mg/l as CaCO3 is

b The alkalinity of the water in mg/l as CaCO3 is

Lime and soda-ash treatment is selected to reduce hardness to practical limits Answer the following questions:

c The amount of lime (mg/l) needed is

d The amount of soda ash (mg/l) needed is

e The concentration of Na (mg/l) in the treated water is

CO2 Ca++ Mg++ Na+

K+

Cl

-HCO3- SO4

0 0.36

5.2 4.5

5.2 2.2 3.7

5 (4 pts) In a sorption isotherm experiment, 1.5 g activated carbon is mixed with 50 ml

methylene blue solution with a concentration of 70 mg/l After equilibrium, the

concentration of methylene blue in the solution was found to be 34 mg/l Assuming no adsorption of methylene blue on the container walls and that sorption is linear What is the value and units of the equilibrium sorption distribution coefficient (K)?

6 (4 pts) A single-stage trickling filter receives wastewater with a BOD of 220 mg/l The

filter is 180-ft in diameter and 7-ft-deep The design flow rate is 2.0 mgd Calculate the effluent BOD if the wastewater temperature is 25 oC

Q=2.0 mgd BOD= 220 mg/l

Q=2.0 mgd BOD= ? mg/l Recirculated flow= 0.5 mgd

7 (5 pts) The schematic diagram below shows part of a wastewater treatment plant The

primary settling tank removes 30% BOD and 60% suspended solids (SS) while the biological treatment removes 90% BOD and 90% SS The characteristics of the

wastewater are indicated at point A

A The primary settling tank is usually designed assuming

(i) Type I settling (ii) Type II settling

(iii) Type III settling (iv) Type IV settling

B The suspended solids (mg/l) entering the aeration tank (point B) is approximately

C The expected BOD (mg/l) at point D

D What is the food-to-microorganisms ratio inside the aeration tank?

E What is the sludge age?

R =30%

Q = 400 m3/d

BOD5=350 mg/l

SS=250 mg/l

Primary settling tank

Aeration tank

settling tank

Waste sludge= 7m3/d

•E SS=10,000 mg/l

•A